// Protocol Buffers - Google's data interchange format
// Copyright 2008 Google Inc.  All rights reserved.
// https://developers.google.com/protocol-buffers/
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package com.google.protobuf;

/**
 * Grab-bag of utility functions useful when dealing with RPCs.
 *
 * @author kenton@google.com Kenton Varda
 */
public final class RpcUtil {
    private RpcUtil() {
    }

    /**
     * Take an {@code RpcCallback<Message>} and convert it to an {@code RpcCallback} accepting a
     * specific message type. This is always type-safe (parameter type contravariance).
     */
    @SuppressWarnings("unchecked")
    public static <Type extends Message> RpcCallback<Type> specializeCallback(
            final RpcCallback<Message> originalCallback) {
        return (RpcCallback<Type>) originalCallback;
        // The above cast works, but only due to technical details of the Java
        // implementation.  A more theoretically correct -- but less efficient --
        // implementation would be as follows:
        //   return new RpcCallback<Type>() {
        //     public void run(Type parameter) {
        //       originalCallback.run(parameter);
        //     }
        //   };
    }

    /**
     * Take an {@code RpcCallback} accepting a specific message type and convert it to an {@code
     * RpcCallback<Message>}. The generalized callback will accept any message object which has the
     * same descriptor, and will convert it to the correct class before calling the original callback.
     * However, if the generalized callback is given a message with a different descriptor, an
     * exception will be thrown.
     */
    public static <Type extends Message> RpcCallback<Message> generalizeCallback(
            final RpcCallback<Type> originalCallback,
            final Class<Type> originalClass,
            final Type defaultInstance) {
        return new RpcCallback<Message>() {
            @Override
            public void run(final Message parameter) {
                Type typedParameter;
                try {
                    typedParameter = originalClass.cast(parameter);
                } catch (ClassCastException ignored) {
                    typedParameter = copyAsType(defaultInstance, parameter);
                }
                originalCallback.run(typedParameter);
            }
        };
    }

    /**
     * Creates a new message of type "Type" which is a copy of "source". "source" must have the same
     * descriptor but may be a different class (e.g. DynamicMessage).
     */
    @SuppressWarnings("unchecked")
    private static <Type extends Message> Type copyAsType(
            final Type typeDefaultInstance, final Message source) {
        return (Type) typeDefaultInstance.newBuilderForType().mergeFrom(source).build();
    }

    /**
     * Creates a callback which can only be called once. This may be useful for security, when passing
     * a callback to untrusted code: most callbacks do not expect to be called more than once, so
     * doing so may expose bugs if it is not prevented.
     */
    public static <ParameterType> RpcCallback<ParameterType> newOneTimeCallback(
            final RpcCallback<ParameterType> originalCallback) {
        return new RpcCallback<ParameterType>() {
            private boolean alreadyCalled = false;

            @Override
            public void run(final ParameterType parameter) {
                synchronized (this) {
                    if (alreadyCalled) {
                        throw new AlreadyCalledException();
                    }
                    alreadyCalled = true;
                }

                originalCallback.run(parameter);
            }
        };
    }

    /**
     * Exception thrown when a one-time callback is called more than once.
     */
    public static final class AlreadyCalledException extends RuntimeException {
        private static final long serialVersionUID = 5469741279507848266L;

        public AlreadyCalledException() {
            super("This RpcCallback was already called and cannot be called multiple times.");
        }
    }
}
